Light pen input systems provide increased ease and convenience of use for computer operators. Input systems of this type require little or no training and enable the operator to quickly and easily interact with the computer. Instead of laboriously typing commands and data into the machine with a conventional keyboard, the operator can simply point to prompts supplied by the display, or write the data to be entered directly on the screen. Unfortunately, typical light pen systems are expensive, provide limited resolution and feel awkward to use.
Conventional light pens have a lens system in the tip of the pen to image a small region of the display screen upon a light sensor in the pen. When the raster scanning electron beam of the display reaches the small region of the display imaged by the lens system, light emitted by the screen phosphor in that small region is detected by the light sensor in the pen. The pen immediately signals the computer system that light has been sensed. Since the position of the light pen at that time coincides with the position of the raster scanning electron beam, the computer can easily determined where the pen is located (i.e. at the location of the scanning electron beam when the light detection signal is received by the computer). Unfortunately, the lens system not only adds to the pen's cost, it also makes the pen bulky so that it has an unnatural and awkward feel.
Light pen designs have been proposed which avoid some of these difficulties, but have other drawbacks instead. In particular, light pens have been proposed which include fiber optic elements so that the need for a lens system can be avoided. U.S. Pat. No. 3,498,692 issued to Jewitt et al, for example, describes a light pen design, which features use of a fiber optic element for collecting light from a cathode ray tube display. The light collected when the pen is placed on the screen is correlated with information relating to the electron beam sweep that produced the display so that the pen location can be identified.
However, while fiber optic pens are lower in cost, more reliable, and provide a better feel, they suffer from poor resolution. Due to the wide angle of acceptance of the fiber optic element, light is collected from a large number of screen picture elements (pels), which reduces the system's ability to accurately correlate the collected light with the display sweep. This in turn reduces the system's ability to identify the pen's position coordinates; that is, location on the screen and accordingly, the pen's resolution.
Because determination of light pen position depends upon collected light, it is important that only relevant light be considered; that is, light collected when the pen is placed on the display screen. Thus, it is desirable for a light pen to include means for determining when the pen has engaged the screen.
Conventional light pen systems rely upon the collected light used for screen coordinate position determination to also provide an indication of screen engagement. This is not very suitable for some applications, such as for hand writing input. For such applications, it is desirable to have a light pen system which can sense screen engagement even in a dark portion of the screen, independently of any particular image on the display.